(a) Field of the Invention
The present invention relates to a method for preparing high molecular weight polycarbonate, more particularly to a method for preparing high molecular weight polycarbonate, which can maximize molecular weight increase of polycarbonate after solid state polymerization and remarkably reduce time required for preparing polycarbonate of the same molecular weight, by lowering mole fraction of arylcarbonate existing in the mixture of polycarbonate prepolymer, end groups of reaction by-products of which the degree of polymerization is less than 3, and unreacted diarylcarbonate through condensation polymerization.
(b) Description of the Related Art
Polycarbonate resin has superior heat resistance, impact resistance, mechanical strength, transparency, etc., and thus it is widely used for preparation of compact disc, transparent sheet, packaging material, automobile bumper, UV shielding film, etc. and its demand is in rapid increase.
Conventional polycarbonate preparation processes are divided into an interfacial polymerization process that uses phosgene and a melt polymerization and a solid state polymerization which do not use phosgene.
The interfacial polymerization process is to mix an aqueous solution of aromatic hydroxy compound such as bisphenol A with gas phase phosgene in an organic solvent to proceed polymerization in the interface between aqueous solution layer and organic solvent layer. Although this process can comparatively easily produce high molecular weight polycarbonate resin in continuous process, since it uses pollutant chlorine type solvent and toxic gas, it is very dangerous and thus enormous equipment cost is required. Meanwhile, the melt polymerization is to proceed polymerization while melting raw material, monomers. Although it has little danger because it does not use toxic substance, in order to produce high molecular weight polycarbonate for injection and extrusion, high temperature and high vacuum equipment is required when treating high viscous reactant and thus quality is lowered. In last, the solid state polymerization is to crystallize low molecular weight polycarbonate prepolymer and then proceed polymerization at temperature range lower than melting temperature. Since it does not use toxic substance and proceeds reaction in solid state, it can prevent deterioration.
However, since so far reported solid state polymerizations (U.S. Pat. Nos. 4,948,871 and 5,214,073) pass crystallization and solid state polymerization processes without removing unreacted diarylcarbonate and end groups of side-products of degree of polymerization less than 3 existing together with semi-crystalline prepolymer of comparatively low molecular weight (weight average molecular weight; 2,000˜20,000 g/mol. Example maximum molecular weight; 15,000 g/mol) before solid state polymerization, molecular weight is at most 38,800 g/mol even after operating for more than 8 hours, due to large mole ratio difference between aromatic hydroxy group and arylcarbonate group. In addition, since such a mole ratio difference increases for enhancement of physical properties of polycarbonate resin, as molecular weight of prepolymer increases. Even if it is effective for physical properties, it has a bad influence on increase in molecular weight during solid state polymerization for the above mentioned reason. Other solid state polymerization processes (U.S. Pat. Nos. 5,717,056 and 5,905,135), for the above mentioned reason, conduct solid state polymerization using comparatively low molecular weight (viscosity average molecular weight: 4,800˜18,000 g/mol) semi-crystalline prepolymer and thus molecular weight increase is at most 31,000 g/mol even after operating for 10 hours after compensating catalyst.
Accordingly, there is a need for studies for polycarbonate preparation process that can prevent deterioration and prepare high molecular weight polycarbonate within a short time.